Add new syntax match group "clojureStringEscape"

Allow for highlighting of escape characters contained within
clojureString.

Also, update clojureCharacter to support all unicode values.
Remove old \u char match

Also, replace hex char range with \x class since it's faster
Remove unneeded patterns from String and Regexp

These appears to be cargo culted from the C syntax file which defines
multibyte L"" string literals for C++. What the skip=/\\\\/ is for, I
still don't know, but it appears to be useless. I will assume it was
similarly cargo culted, and wait for bug reports.